A secret list of suspected terrorists banned from flying to or within the US has more than doubled to 21,000 in the past year.

âUnderwear bomberâ Umar Farouk Abdulmutallab (Picture: AP)

Names of the no-fly list rocketed after the failed bombing of a Detroit-bound jetliner by âunderwear bomberâ Umar Farouk Abdulmutallab in 2009.

Despite the elimination of many senior members of al-Qaeda, the US government believes the terror threat extends beyond the group responsible for the 9/11 attacks in 2001.

Transport security boss John Pistole said: âBoth US intelligence and lawÂ enforcement communities and foreign services continue to identify people who want to cause us harm, particularly in the US and particularly as it relates to aviation.â

Only 500 names on the list are American. Washington has refused to reveal who or why anybody is included but is believed to have lowered the standard for suspects and then scoured files for anyone who qualified.

The American Civil Liberties Union has sued the US government on behalf of passengers who have not been able to fly for work purposes or to see their family.

Blacklisted individuals cannot defend themselves if bureaucrats refuse to tell them why they cannot board aircraft, the group says.

Complainants can write to homeland security officials â but the only way to find out if they are still on the list is to try to fly again.

Abdulmutallab tried to detonate explosives hidden in his underwear on board a Northwest Airlines flight from Amsterdam to Detroit on Christmas Day 2009.

The Nigerian-born 25-year-old, who claimed he was trained by al-Qaeda in the Arabian Peninsula, is to be given a life sentence on February 16.